FIGS. 1A and 1B illustrate an exemplary structure of a network including switch stacking. The switch stacking is a technology to connect multiple switches (layer 2 switches) by stack connection to represent the multiple switches as one switch. The switch stacking may also be referred to as switch virtualization. FIG. 1A illustrates an exemplary physical structure of the network. Referring to FIG. 1A, four switches SW-A to SW-D are connected by stack connection through stack links L1 to L4. FIG. 1B illustrates an exemplary logical structure of the network illustrated in FIG. 1A. Referring to FIG. 1B, multiple switches connected by stack connection are logically represented as one switch V. In this case, a user may manage the four switches SW-A to SW-D integrally (as one switch V).
On the other hand, a technology called link aggregation is in use, in which multiple physical links (lines) are grouped to be logically handled as one link. Use of the link aggregation allows the communication bandwidth to be improved and robustness due to redundancy to be ensured. The group of the physical links logically handled as one link is called a link aggregation group (LAG).
When the switch stacking is applied to multiple switches, the multiple switches are logically handled as one switch. Accordingly, links belonging to the same link aggregation group may be set across multiple switches.
FIG. 2 illustrates an example of link aggregation employed for switches to which the switch stacking is applied.
Referring to FIG. 2, the switch SW-B and the switch SW-C, among the four switches SW-A to SW-D to which the switch stacking is applied, are connected to a node A through different physical links. When the link aggregation is employed for the two links in this example, the node A and one logical switch (a switch stack) including the switches SW-A to SW-D are logically handled as if the node A were connected to the switch stack through one link (link aggregation group G).
However, as apparent from FIG. 2, connecting multiple switches to the same link aggregation group G physically forms a network loop. Accordingly, a frame storm may occur in the absence of appropriate handling.
Conventional methods of handling the network loop include a control method using spanning tree protocol (STP) and a control method using frames including a time to live (TTL) value.
FIG. 3 illustrates an exemplary control method using the STP. In the STP, the control is performed so that the network loop is not formed by appropriately blocking a redundant path. Referring to FIG. 3, a port connected to the stack link L4 is blocked in each of the switch SW-A and the switch SW-D.
FIG. 4 illustrates an exemplary control method using frames including a TTL value. In this method, a TTL value is included in each frame and the TTL value is decremented each time the frame passes through one switch. When the TTL value decreases to zero, the frame is discarded. As a result, the frame is prevented from being infinitely circulated over the network and the loop is substantially eliminated. In the example in FIG. 4, a frame F1 including a TTL value of one is transmitted in a direction from the switch SW-D to the switch SW-A and a frame F2 including a TTL value of two is transmitted in a direction from the switch SW-D to the switch SW-C. The frame F1 is not transferred to the stack link subsequent to the switch SW-A, and the frame F2 is not transferred to the stack link subsequent to the switch SW-B.
Japanese Laid-open Patent Publication No. 2008-236212 discloses a related technique.